It is known to isomerize lower alkyl-substituted aromatic hydrocarbons at elevated temperatures by passing the vaporized feed into contact with a solid silica-alumina catalyst composite for the purpose of altering the isomer distribution of the hydrocarbon feed. The usual objective is to convert the feed into a mixture of isomeric compounds which more nearly approximates the known isomer equilibration composition for the specific feed composite or mixture. In practice, the foregoing equilibration process is coupled with a subsequent separation process utilizing physical or chemical means for the preferential and selective removal of a desired isomer from the equilibrate. By recycle of the residue from the separation stage and repetition of the overall operation one is able to convert a single isomeride or isomeric mixture to a desired specie of the equilibration set.
An undesirable aspect in conventional practice in the foregoing staged conversion and recovery cycle is that a substantial fraction of the process feed is lost because of a concurrent disproportionation reaction. For example, in a process for the production of p-xylene in which a p-xylene-reduced xylene mixture is isomerized in an extinction recycle process, the formation of disproportionation products such as benzene, toluene, and trimethyl benzenes represents an appreciable loss of xylene product or a large process burden where these components of the product stream are recycled to the process together with fresh feed. In a typical petroleum refinery operation it is usually preferred not to recycle such by-products. Consequently, as much as 30-40% of the xylene feed is converted through the disproportionation reaction to less desirable product.